In recent years, boron-neutron capture therapy (BNCT) attracts attention as a new therapeutic method for cancer by utilizing a radioisotope. In boron-neutron capture therapy, a boron compound containing a 10boron isotope (10B) is incorporated into a cancer cell, which is then irradiated with a low-energy neutron ray (for example, thermal neutron) to destruct the cancer cell locally by a nuclear reaction occurring in the cell. In this therapeutic method, the selective accumulation of a 10B-containing boron compound in cells of cancer tissue is critical in enhancing the therapeutic effect, thus it is necessary to develop boron compounds that is incorporated selectively into cancer cells.
Conventionally, boron-containing compounds having a boron atom or a boron atomic group introduced into their basic skeleton have been synthesized as drugs used in BNCT. Clinically used drugs include p-boronophenylalanine (BPA) and mercaptoundecahydrododecaborate (BSH). Among these drugs, BSH is used mainly in treatment of a brain tumor in the form of a sodium salt and confirmed to be useful (see, for example, Non-Patent Documents 1 to 8).    Non-Patent Document 1: I. M. Wyzlic et al., Tetrahedron Lett., 1992, 33, 7489-7490.    Non-Patent Document 2: W. Tjark, J. Organomet. Chem., 2000, 614-615, 37-47.    Non-Patent Document 3: K. Imamura et al., Bull. Chem. Soc. Jpn., 1997, 70, 3103-3110.    Non-Patent Document 4: A. S. Al-Madhorn et al., J. Med. Chem., 2002, 45, 4018-4028.    Non-Patent Document 5: F. Compostella et al., Res. Develop. Neutron Capture Ther., 2002, 81-84.    Non-Patent Document 6: S. B. Kahl et al., Progress in Neutron Capture Therapy for Cancer, Plenum Press, New York 1992, 223.    Non-Patent Document 7: J. Cai et al., J. Med. Chem., 1997, 40, 3887-3896.    Non-Patent Document 8: H. Lim et al., Res. Develop. Neutron Capture Ther., 2002, 37-42.